Piston assembly for pumps, motors and the like

ABSTRACT

A dry-running piston assembly, e.g., for compressor pistons, in which the piston is slidable with slight clearance in a cylinder and is formed with at least one groove confronting the wall of the cylinder and receiving a wear-type (e.g., self-lubricating) piston ring. The piston ring is biased outwardly into contact with the wall of the cylinder and is provided with shoulders engageable with shoulders of the groove to prevent further outward displacement of the ring beyond a predetermined wear condition. The ring may be composed of polytetrafluoroethylene or another low-friction or self-lubricating material.

limited States Patent 11 1 3&50919 Miiller [45] Apr, 10, 1972 [s41 PISTON ASSEMBLY FOR PUMPS, FOREIGN PATENTS OR APPLlCATlONS MUTORS AND THE LHKE 900,921 10/1944 France ..277/168 [72] Inventor: lllans-Jiirg Miiller, Surth, Germany 1,488,517 6/1967 France ..277/ 178 287,694 10/1915 Germany ..277/168 [73] Ass1gnee. Lmde Aktlengesellschait, Wresbaden, Ger- 276,546 9/l927 Great Britain D "277/168 many 176,703 7/1935 Switzerland ..277/168 [22] Filed: June 9, 1970 Primary Examiner-Edgar W. Geoghegan [2]] App! 45391 Assistant ExamineF-lrwin C. Cohen Attorney-Karl F. Ross [30] Foreign Application Priority Data June 11, 1969 Germany ..P 19 29 630.1 [571 ABSTRACT A dry-running piston assembly, e.g., for compressor pistons, in Cl which the piston is slidable with slight clearance in a cylinder 92/253, 92/255, 277/168, 277/1 8 and is formed with at least one groove confronting the wall of [51] Int. Cl ..F16J 9/06, F16 9/08, F163 9/20 the cylinder and receiving a weamype (egg selmubricating) [58] Field Of Search...- ..277/9, ll, 168, 162, 178; piston ring The piston i is biased outwardly into Contact 29/1565; 92/193 185 7 with the wall of the cylinder and is provided with shoulders en- 255 gageable with shoulders of the groove to prevent further outward displacement of the ring beyond a predetermined wear [56] References Cmd condition. The ring may be composed of UNITED STATES PATENTS pfiytgtfifluogoetlglene or another low-friction 0| selfu 1 a n m er 1,365,578 1/1921 Zippler ..92/182 g 2,009,958 7/1935 l-lance ..92/255 X 4 Claims, 12 Drawing Figures I le 3 a, s u ,9 |0

s $1 W 13 lo i/ s 6O s lb f I 16 8 1c 9 I9 l 3/ l5 PATENTEDAPRIBIQR 3,656,414

sun-113m s ",F 'IG.4 w .Fl Gf6 v INVENTOR.

HANS -J6Re [MULLER War! ATTORNEY PATENTEDAPR 18 1912 SHEETRUF5 L 1 I a m I0 32 E 32 Z TIA f U I FIG.8

96? g. e ATTORNEY PISTON ASSEMBLY FOR PUMPS, MOTORS AND TIE LIKE FIELD OF THE INVENTION My present invention relates to dry-running piston assemblies and, more particularly, piston arrangements for motors, compressors, pumps and the like in which lubrication is effected at a solid-solid interface by the use of a self-lubricating material. More particularly, the invention relates to dryrunning piston compressors, the piston of which is slidable with slight clearance in a cylinder and is fonned with sealing or piston rings engageable with the cylinder wall.

BACKGROUND OF THE INVENTION In dry-running machines, especially dry-running piston machines, an external lubricant of the flowable type is avoided and there exists a solid-solid interface between the moving parts at which sealing must be effected. Lubricating-film machines, by contrast, provide means for entraining a more or less viscous lubricant film between the moving parts, e.g., the piston and the cylinder wall, so that the film forms a seal between the two and friction occurs not at a solid-solid interface, but between two solid-liquid interfaces and between shear planes in the liquid film. Dry-running machines of the aforedescribed type, however, have the advantage that lubrication problems involving fluids can be avoided.

For the most part, dry-running piston machines, especially dry-running compressors, comprise a reciprocable piston which is guided in a cylinder with clearance between the outer surface of the piston head and the cylinder wall.

The gap between the piston head and the cylinder wall is sealed by one or more rings carried by the piston and generally of a wearable material, e.g., a self-lubricating substance or a low-friction material which, in turn, is filled with self-lubricating substances. Typical of such rings are those composed of polytetrafluoroethylene (Teflon) which have extraordinarily low coefficients of sliding friction, high thermal stability and satisfactory wear properties for most compressive purposes.

The polytetrafluoroethylene ring may be replaced by a ring of another material provided with a low-friction, self-lubricating packing or a Teflon ring may be formed with such a packing in accordance with various teachings of the prior art.

The self-lubricating materials which have been proposed heretofore in this connection include molybdenum disulfide which transfers to the surfaces of the relatively moving parts and appears to effectively smooth the surfaces. Other solid lubricants which may be used as or in self-lubricating systems are graphite and mixtures of molybdenum disulfide and graphite and mixtures of finely divided graphite, molybdenum disulfide and polytetrafluoroethylene.

It has been the practice heretofore, when using wearable rings of the aforedescribed type, to provide means for biasing the rings outwardly against the cylinder wall; such means has included metal spines or backings for the rings, spring arrangements for urging the piston rings outwardly etc. In practice, these systems have been found to be disadvantageous in that they frequently result in a direct contact between the metal member and the wall of the cylinder upon excessive wear of the piston ring, or to canting'of the ring and jamming of the latter with excessive wear.

While it has been proposed to avoid this disadvantage by providing coacting stop means upon the piston ring and the piston for preventing excessive outward movement of the ring, arrangements of the latter type have necessitated the construction of the piston from several points at high costs. Another disadvantage of the composite piston system is, moreover, the elevatedmass which must be provided in such pistons with substantial technological disadvantage.

OBJECTS OF THE INVENTION It is, therefore, the principal object of the present invention to provide an improved piston assembly for dry-running piston machines and especially compressors or gas-displacement pumps in which the aforementioned disadvantages are avoided.

Another object of my invention is to provide an improved dry-running piston machine in which the danger of scoring the cylinder wall is eliminated but the cost of manufacture of the piston is maintained at a minimum and the piston mass is likewise held low.

Still another object of the invention is to provide an. improved method of making a piston assembly.

SUMMARY OF INVENTION These objects and others which will become apparent hereinafter are obtained, in accordance with the present invention, with a piston assembly for a dry-running machine especially a piston-type gas-displacement pump or compressor, in which the piston body is received with clearance in a cylinder and is formed along its periphery with one or more low-friction or self-lubricating sealing rings, biased outwardly under the spring or fluid pressure.

According to essential principles of this invention, the piston head is formed in a single piece, i.e., integrally or unitarily from a single body of metal and is not assembled from a number of pieces, this monolithic piston head being formed with outwardly open circumferential grooves receiving the piston rings. The grooves are provided with catchers engageable with annular shoulders formed on the piston ring and preventing excessive outward displacement of the piston rings, i.e., outward displacement under the spring or fluid pressure beyond a level which is safe with respect to damage to the surfaces of the machine. There is, therefore, no danger that the ring-supported spine or spring will score the walls of the cylinder.

It is an important feature of this invention that the ringreceiving grooves mentioned earlier are formed in the piston head by a material-removal or cutting operation, e. g., turning of the piston head on a lathe and, at least over part of the circumferential length of each groove, the groove has a re-entrant portion defining the abutment for a corresponding shoulder. At least over part of the circumferential length of each groove, therefore, the shoulder of the ring bears directly against the body of piston material flanking the groove.

The one-piece construction of the piston and the formation of the grooves in a single member not only reduces the manufacturing cost, the cost of material and the weight of the piston for particular piston volume or set of dimensions, but also provides a catcher-type of lost-motion engagement of the piston rings to prevent damage to the cylinder walls.

According to a more specific feature of this invention, the groove is flanked by at least one ledge extending over a major portion of the circumferential length of the groove and cooperating with a corresponding shoulder of the piston for engagement therewith upon wear of the annular projection of the piston ring which extends through the groove into engagement with the wall of the cylinder. The circumference of the piston may be cut away at one or more locations to permit tangential insertion of the piston rings. The ledge configuration and the profile of the piston ring preferably are of L-shape or T-shape as will be apparent hereinafter.

In the region in which the wall of the groove is cut away to accommodate tangential insertion of the piston rings, I may provide any one of a number of securing devices engageable with the shoulder of the piston ring for retaining the latter in place.

I have found, more surprisingly, that the present invention enables the light-weight construction of the piston head to the point that it is even possible to provide catcher-ring arrangements and certain pistons for which such piston-ring structures have not heretofore been considered.

BRIEF DESCRIPTION OF DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description. reference being made to the accompanying drawing in which:

FIG. 1 is an axial cross-sectional view through a portion of a machine, i.e., a dry-running piston pump or compressor, according to the invention;

FIG. 2 is a cross-sectional view along the line II-II of FIG. 1;

FIG. 3a is a side-elevational view, partly broken away of the groove periphery of the piston of FIGS. 1 and 2;

FIG. 3b corresponds to a detail view of a portion of the wall structure of FIG. 1 or an axial cross section through the view of FIG. 30;

FIG. 4 is a detail view through yet another piston structure according to the invention;

FIG. 5 is a transverse cross-sectional view through a portion of this structure, the section plane being illustrated at V-V in FIG. 6;

FIG. 6 is an elevational view of the retaining region of the piston;

FIG. 7 is an axial cross-sectional view of still another piston arrangement, illustrating only a portion of the piston head proximal to the cylinder wall;

FIG. 8 is a transverse section through part of FIG. 7 along the line represented at VIII-VIII of FIG. 9;

FIG. 9 is an elevational view of this region;

FIG. it) is an axial cross-sectional view of a further embodiment ofthis invention; and

FIG. 11 is a cross section taken along the line XI-XI of FIG. 10.

SPECIFIC DESCRIPTION In FIGS. 1, 2, 3a and 312, I have shown a piston assembly for a linearly reciprocable dry-running piston-type compressor in which the piston assembly comprises a one-piece integral or unitary head I, mounted upon a piston rod 2 by a threaded connection In and bearing upon a shoulder 20 of this rod. The rod 2 is linearly slidable in a bushing 4 of a cylinder head 3 with small play. It should be noted that the concept ofsmall play, as here used, is intended to describe the minimum play consistent with sliding movement of the piston in the cylinder head. In place ofthe slide bearing 4, the piston head 1 may be provided with a guide ring as shown at 4a in FIG, 1 to limit the lateral movement of the piston head within the cylinder 5a as indicated. The limited play and lack of lateral movement resulting from the use of the bearing 4 and/or the use of the guide ring 40 is designed to prevent the piston body from engaging and gouging the cylinder wall 6 during reciprocation of the piston and in the event the sealing rings are worn to the point that they no longer engage the wall.

The piston head I is, as shown in FIG. 1, hollow and provided with a weight-reducing cavity 1d as well as with a central bore 1b which is stepped to form a ledge adapted to seat against the shoulder 2a. A clamping nut la is threaded into the stepped upper end of the piston, proximal to the cylinder head 3, to engage the thread of rod 2 and lock the piston head to the latter.

Along its outer periphery, the piston head 1 is formed with four axially equispaced circumferential channels, 7, lying in planes perpendicular to the axis of the piston and in the plane of the paper as illustrated in FIG. 2. Each ofthese channels is of generally T-shaped cross section and comprises an outer channel portion 9 of small axial width confronting the wall 6 of the cylinder 5 and, inwardly thereof, a channel portion 8 of greater axial width so that, flanking the channel portion 9, are inwardly turned ledges 10 with radial abutment surfaces 10a constituting stops for the piston rings which are received in these channels.

In the embodiment illustrated in FIGS. 1, 2, 3a and 3b, two such ledges and stop surfaces are provided; it is also contemplated within the scope of the present invention to provide channels of L-shaped section wherein only one such ledge may be necessary. An arrangement of this type has been described in connection with FIGS. 10 and l1.

The piston rings 11 which are received in the channel 7 are of T-shaped cross section (see especially FIGS. 1 and 3) and are composed of a dry-running material, especially a dry lubricant such as fiber packing impregnated with graphite and/or molybdenum disulfide or are composed of a synthetic-resin material with low coefl'icient of sliding friction and selflubricating properties, preferably polytetrafluoroethylene (Teflon). Alternatively, the ring may be composed of a lowfriction synthetic resin (e.g., Teflon) impregnated with or combined with a solid lubricant such as molybdenum disultide.

It is important for the present invention, in any case, that the piston rings be constituted of a material capable of maintaining a sealing fit with the cylinder wall 6, wearable to accommodate itself to any wall contours without breaking the seal, having a low coemcient of sliding friction to restrict drag, characterized by inertness with respect to the fluid contained in the piston chamber, nonseizing with respect to the material of the wall of the cylinder, and having a hardness less than that of the wall material. In the case in which the cylinder wall is composed of cast iron or steel, the piston rings may invariably be constituted of polytetrafluoroethylene and, where hardened wall surfaces are concerned, a sintered bronze or other softer metal impregnated with a solid lubricant such as molybdenum disulflde.

According to the principles of the present invention, the piston ring 11 comprises a shank 11a with a radial width W which is in excess of the radial width w of the ledges 10. Moreover, the radial width s of the head of the T is less than the radial width S of the channel portion 8 accommodating same, thereby creating a pair of annular shoulders 12 at the junction of the shank with the head adapted to seat against the abutment surfaces 10a mentioned earlier. The axial width a of the head of the T is substantially equal to that of the channel portion 8 to prevent canting of the piston ring within this channel.

Behind the piston ring 11, I provide an outwardly loaded spring 13 of band spring steel of conventional configuration to urge the piston ring radially outwardly and press the annular face 11b of the shank against the wall 6 of the cylinder. When a spring of this type is employed, the compressed thickness I of the spring should be dimensioned such that S l+s and W w+r where r represents the gap between the piston head 1 and the cylinder wall 6.

In place of the spring bias illustrated, or in addition thereto, fluid pressure ahead of the piston may be employed to urge the piston rings outwardly. In this case, the piston head 1 is provided with channels communicating between the working compartment of the piston (i.e., the space in the cylinder ahead of the piston) and the channel section 8 behind the head of the T. In FIG. 31;, for example, one such passage is shown at 14 to communicate between the space 8a behind the spring 13 of the uppermost piston ring 11 and the chamber 5a ahead of the piston via a narrow gap 6a between the piston head 1 and the wall 6. At the lower end of the piston a similar passage is formed at 15 and is effective when the piston operates in a double-acting mode to compress fluid on opposite sides of the piston head with each stroke and during each half cycle.

With wear of the shank portion of the piston ring 11, the springs 13 and the fluid behind the springs urge the piston rings 11 outwardly into engagement with the wall 6 and abrade in frictional engagement with this wall as the piston reciprocates. With excessive wear of the piston rings 11, however, the shoulders 12 come to bear against the abutments l0 and prevent movement of the head of the piston ring from the channel portion 8. As a consequence, the remainder of the piston ring cannot enter the gap between the piston and the wall 6, nor can the steel spring 13 project from the channel into engagement with the wall for possible scoring thereof. The piston rings 11, provided with the annular shoulders 12, guarantee, in combination with the guide 4, a minimum play between the piston head and the cylinder surface 6 and, therefore, a labyrinth seal between piston head and cylinder.

The piston rings III are inserted into the grooves or channels 7 at regions 17 in which the ledges are cut away (FIG. 2) over a sufficient portion of the circumferential length to enable the piston ring to be inserted generally tangentially into the channel as represented in dot-dash lines in FIG. 2.

To lock the piston rings in place, I provide retaining means overhanging the cut-away portions of the channels or the windows" through which the piston rings are inserted, such means including rings, also a stop S is engaged between the ends of the split piston to prevent rotation of the piston ring relatively thereto about the piston axis, shoulders, pins, abutments or the like. In the embodiment illustrated in FIGS. 1, 2, 3a and 3b, for example, I provide along the row of windows 17, in the circular ribs 18 which project radial from the piston head 1 and separate the channels 7 from one another, respective cylindrical bores 19a which, as best seen in FIG. 3a, have diameters D in excess of the width d of the web or rib 18 in the region of the windows. In the wall of the bore 19a there is provided a circular, inwardly open recess 19 with a seating surface 20 for a snap ring 21 which spring outwardly into this recess when its inwardly deflectible lobes 21a are released. The recess 19 is so arranged that generally sectoral portions 21b and 210 overhang the cut-away portions of the groove 7 and serve to intercept the piston ring 11 should the latter tend to move outwardly in this region. To this end, the inner surface Zld of the ring lies precisely flush with the imaginary cylinder described by the generatrices of the cylindrical surfaces 10a mentioned earlier. Hence even in the region of the windows, an abutment is provided for the shoulders 12 of the piston ring.

The piston rings 11 are, of course, of the split type and, preferably, the gaps between the ends of the piston rings are angularly staggered from one piston ring to the other along the piston head.

In FIGS. 4-6, I have shown another embodiment of the present invention and both in these Figures and in FIGS. 7-9 parts which are functionally identical to those of FIGS. 1, 2, 3a and 3b are given the identical reference numerals and various elements, to the extent that they will correspond to those of FIG. 1, have been omitted to simplify the description. In the system of FIGS. 4-6, the abutment in the window 17 for the piston rings 11 is formed by cap screws 26 as best shown in FIGS. 5 and 6. The webs or ribs 18 are here provided with threaded radial bores 27, of small diameter, substantially midway between the flanks 17a and 17b of the respective windows of the adjacent channels 7. Again the abutment formed by the cap screw must overhang the flanks 17a and 17b and, for this reason, the recess 29 is formed as a countersink in the rib 18 concentric with the threaded bore 27. Again, the diameter D of this recess exceeds the width d between the flanks 17a and 17b. The cap screw 26 has a head 28 of circular configuration and of a diameter close to the diameter D so that sectoral portions 28a and 28b delineated by dot-dash lines in FIG. 6, project into the windows 17. The inner surface 28a of these sectors form abutments which lie along the imaginary cylindrical surface described by the generatrices for the surfaces 10a of the ledges 10. Hence the recess 29 may have a radial thickness .r w. In FIGS. 7-9, a further arrangement is illustrated. Here again, the fastening means for partially obstructing the window 17 is wholly received in the piston head 1 within the surface outlined at the latter. In this embodiment, however, recesses 33 of rectangular outline are formed over substantially the entire circumferential arc of the windows 17 and the fastening means includes rectangular plates 30 which are fastened to the piston head by counter sunk screw 32 threaded into radial bores 32 in the webs 13 between the channel 7. The axial width D of these plates exceeds the axial width d of the web 18 in the region of the windows so that the plates overhang these windows and block outward movement of the piston rings 11. The inner surfaces 30a of the plates lie flush with the corresponding surfaces 10a of the ledges 10 whereby the radial thicknesses of the plates 30 should be at most equal to the radial thicknesses of these ledges.

In FIGS. Ill and 11, I have shown still another embodiment of the invention wherein, in place of stepped channels as represented in FIGS. 1-9, the channel 41 extending circumferentially upon the piston head 40 has substantially unifonn radial cross section. In this embodiment, however, the T- shaped piston rings 43, of the type described with respect to the piston rings 11 previously, are urged radially outwardly by band-type springs 13 and are snugly fitted between the flanks 44 and d5 of these channels.

The flanks M and 45 lie in planes perpendicular to the axis of the piston and extend generally radially to the mouth of the channels which confront the cylinder wall (not shown). Close to the mouth of the channels, the flanks 44 and 45 are formed with grooves opening axially toward the piston ring 43 accommodated therein and receiving abutment rings 52, 53 adapted to engage the shoulders of the piston rings 43 as previously noted. The rings 52 and 53 may be constituted as snap rings (one piece) or may be formed from a plurality of ring segments.

In the embodiment illustrated in FIG. 10, the abutment rings 53 are of circular cross section and the grooves 46 receiving same are of substantially semi-circular cross section. The grooves 46, however, have depth y slightly less than the axial thickness Y of the ring so that a portion of each ring extends into the paths of the shoulders 43a of the piston rings. Since T-protile piston rings are used for the two upper channels 41 in the system of FIG. 10, grooves are provided in both flanks of the channel to accommodate abutment rings 52 which engage each of the shoulders. For the lower channel 41', however, only a single groove 51 is provided adjacent the channel mouth and confronting the nongrooved flank. In this embodiment, the piston ring 49 is of L-profile and therefore has only a single annular shoulder 48 serving as an abutment in engagement with the square-section ring 53 which is received within the similarly profile groove 51. The axial thickness Z of this ring exceeds the axial depth 2 of the recess 51 by an amount equal approximately to the height 1. It has been found that the square-profile ring of FIG. 10 has the advantage that a reduced camming action is exerted upon the piston ring by comparison with the circular-section ring also illustrated in this Figure. As shown in FIG. 11, the inner radius L of the piston ring 43, 49 should exceed the radius M of the floor 41a of the groove to enable the piston rings to be shifted eccentrically (FIG. 11) and permit grooves 46, 47 and 51 to be exposed at least at one side (at the left in FIG. 11) so that the rings may be inserted. In this case, no ledges need be cut away.

The improvement described and illustrated is believed to admit of may modifications within the ability of persons skilled in the art, all such modifications being considered within the spirit and scope of the invention except as limited by the appended claims.

Iclaim:

l. A piston, especially for a dry-running compressor, comprising a cylindrical piston head having a unitary ring portion, at least one circumferential groove formed in said unitary ring portion, at least one ledge formed unitarily on said ring portion and partly overhanging said groove while extending circumferentially around said ring portion over the major portion of the circumference thereof while being omitted over a fraction of said circumference to form a single window, a single one-piece split piston ring threaded into said groove through said window and resiliently biased outwardly while having a shoulder engageable with said ledge, said ring projecting from said groove outwardly beyond said ledge, the ends of said ring being closely adjacent one another after insertion, said window having a circumferential length sufficient to enable said ring to be threaded into said groove generally tangentially.

2. The piston defined in claim 1, further comprising retaining means at said window overhanging said groove and engageable with said ring for restricting outward displacement thereof.

3. The piston defined in claim 2 wherein said retaining means includes a snapring removably received in said piston head and having a sectoral portion projecting beyond said window into the path of said shoulder, said piston head being formed with a circular recess accommodating said snapring. 5

4. The piston defined in claim 1 wherein said ring is composed at least in part of a low-friction non-metallic material. 

1. A piston, especially for a dry-running compressor, comprising a cylindrical piston head having a unitary ring portion, at least one circumferential groove formed in said unitary ring portion, at least one ledge formed unitarily on said ring portion and partly overhanging said groove while extending circumferentially around said ring portion over the major portion of the circumference thereof while being omitted over a fraction of said circumference to form a single window, a single one-piece split piston ring threaded into said groove through said window and resiliently biased outwardly while having a shoulder engageable with said ledge, said ring projecting from said groove outwardly beyond said ledge, the ends of said ring being closely adjacent one another after insertion, said window having a circumferential length sufficient to enable said ring to be threaded into said groove generally tangentially.
 2. The piston defined in claim 1, further comprising retaining means at said window overhanging said groove and engageable with said ring for restricting outward displacement thereof.
 3. The piston defined in claim 2 wherein said retaining means includes a snapring removably received in said piston head and having a sectoral portion projecting beyond said window into the path of said shoulder, said piston head being formed with a circular recess accommodating said snapring.
 4. The piston defined in claim 1 wherein said ring is composed at least in part of a low-friction non-metallic material. 